The Pines That Lean Towards The Equator

Most trees grow vertically straight, but under challenging
conditions where individuals have to compete for light, or when
mechanical stress is intense, trees may grow at an angle.

Araucaria columnaris, or Cook pines —named after
Captain James Cook, whose second voyage around the globe carried
the first botanists to classify the tree— is a tree endemic to New
Caledonia in the Melanesia region of the southwestern Pacific
Ocean, but have since been planted in temperate, subtropical, and
tropical areas throughout the world. When grown outside of its
native range, the Cook pines have a pronounced lean that’s so
ubiquitous that it is often used as the identifying characteristic
for the species. But until recently, nobody paid much attention to
which direction it leaned or by how much.

Matt Ritter at the California Polytechnic State University in
San Luis Obispo was researching the Cook pine for an upcoming book
on the urban trees of California, when he realized that the pines
always leaned south. To ascertain whether that’s always the case,
he rang up a colleague in Australia and was surprised to learn that
the trees down under leaned north.

Intrigued, Matt Ritter and his team expanded their efforts and
studied 256 Cook pines scattered across five continents ranging
from latitudes of 7° and 35° north, and 12° and 42° south. They
found that the trees always leaned towards the equator, and the
magnitude of the lean increases the further they went from the
equator. On average, the trees tilt by 8.50 degrees, although one
specimen in Australia was found to be leaning at nearly 40
degrees.

It’s not clear why the Cook pines exhibit this peculiar
behavior, but the researchers feel it’s due to phototropism—the
same phenomenon that causes houseplants to lean towards the sun.
It’s possible the Cook pines bend themselves to better catch the
slanting rays of sunlight at higher altitudes. In most trees, the
tendency to lean towards the sun is counterbalanced by their
sensitivity to the Earth’s gravitational pull, a phenomenon called
gravitropism, that keep trees upright. The researchers speculate
that the Cook pines might be lacking this ability.